A centrifugal compressor is used for a turbocharger or a turbo freezer installed on a vehicle or a ship. The centrifugal compressor includes an impeller rotating at high speed and utilizes centrifugal force to increase the pressure of a fluid. While a wide operating range is required for the centrifugal compressor, decrease in the flow rate of a fluid introduced into the centrifugal compressor leads to a surging, where the discharge pressure pulsates and the operation state becomes unstable, causing the centrifugal compressor to reach the operation limit. Thus, it is necessary to lower the limit flow rate which a surging occurs in order to secure a wide operating range for the centrifugal compressor.
In order to lower the above described limit flow rate, it is effective to swirl the fluid introduced into the impeller in the same direction as the rotational direction of the impeller (forward rotation) as shown in FIG. 13A. Herein, FIG. 13A is a graph showing the change in the performance characteristic of the compressor when the rotation speed is constant and the fluid introduced into the impeller is swirled in the forward direction. As it can be seen from FIG. 13A, when the fluid introduced into the impeller is swirled in the forward direction, the surge line moves toward the left side of the graph, lowering the limit flow rate at which a surging occurs. However, it is impossible to expand the operating range itself because the maximum flow rate also decreases when the fluid introduced into the impeller is swirled in the forward direction.
On the other hand, high pressure ratio is required when the flow rate is high. In order to achieve high pressure ratio, it is effective to swirl the fluid introduced into the impeller to a direction opposite to the rotational direction of the impeller (reverse rotation) as shown in FIG. 13B. Herein, FIG. 13B is a graph showing the change in the performance characteristic of the compressor when the rotation speed is constant and the fluid introduced into the impeller is swirled in the reverse direction. As it can be seen from 13B, when the fluid introduced in to the impeller is swirled in the reverse direction, the performance curve moves to the top-right side of the graph, increasing the pressure ratio. However, the surge line also moves to the right side of the graph when the fluid introduced into the impeller is swirled in the reverse direction, which narrows the operating range itself.
Patent Document 1 discloses a pre-whirl generating device for a centrifugal compressor of an automobile turbocharger, the pre-whirl generating device being capable of swirling the fluid introduced into the impeller in the reverse direction when the flow rate is high and in the forward direction when the flow rate is low by operating a variable mechanism constituted of an air deflection vane with an actuator and the like. According to the pre-whirl generating device of Patent Document 1, it is possible to increase the pressure ratio by introducing a reverse-swirl flow when the flow rate is high and to avoid a surging by introducing a forward-swirl flow when the flow rate is low, as shown in FIG. 14. Also, it is possible to secure a wide operating range because the maximum flow rate of the operating range is not lowered.